Anti-IgLON5 antibodies cause progressive behavioral and neuropathological changes in mice

HLA-DQB1*05 subtypes and not DRB1*10:01 mediates risk in anti-IgLON5 disease

Anti-IgLON5 disease is a rare and likely underdiagnosed subtype of autoimmune encephalitis. The disease displays a heterogeneous phenotype that includes sleep, movement and bulbar-associated dysfunction. The presence of IgLON5-antibodies in CSF/serum, together with a strong association with HLA-DRB1*10:01∼DQB1*05:01, supports an autoimmune basis. In this study, a multicentric human leukocyte antigen (HLA) study of 87 anti-IgLON5 patients revealed a stronger association with HLA-DQ than HLA-DR. Specifically, we identified a predisposing rank-wise association with HLA-DQA1*01:05∼DQB1*05:01, HLA-DQA1*01:01∼DQB1*05:01 and HLA-DQA1*01:04∼DQB1*05:03 in 85% of patients. HLA sequences and binding cores for these three DQ heterodimers were similar, unlike those of linked DRB1 alleles, supporting a causal link to HLA-DQ. This association was further reflected in an increasingly later age of onset across each genotype group, with a delay of up to 11 years, while HLA-DQ-dosage dependent effects were also suggested by reduced risk in the presence of non-predisposing DQ1 alleles. The functional relevance of the observed HLA-DQ molecules was studied with competition binding assays. These proof-of-concept experiments revealed preferential binding of IgLON5 in a post-translationally modified, but not native, state to all three risk-associated HLA-DQ receptors. Further, a deamidated peptide from the Ig2-domain of IgLON5 activated T cells in two patients, compared with one control carrying HLA-DQA1*01:05∼DQB1*05:01. Taken together, these data support a HLA-DQ-mediated T-cell response to IgLON5 as a potentially key step in the initiation of autoimmunity in this disease.

Genetic forms of tauopathies: inherited causes and implications of Alzheimer's disease-like TAU pathology in primary and secondary tauopathies

Tauopathies are a heterogeneous group of neurologic diseases characterized by pathological axodendritic distribution, ectopic expression, and/or phosphorylation and aggregation of the microtubule-associated protein TAU, encoded by the gene MAPT. Neuronal dysfunction, dementia, and neurodegeneration are common features of these often detrimental diseases. A neurodegenerative disease is considered a primary tauopathy when MAPT mutations/haplotypes are its primary cause and/or TAU is the main pathological feature. In case TAU pathology is observed but superimposed by another pathological hallmark, the condition is classified as a secondary tauopathy. In some tauopathies (e.g. MAPT-associated frontotemporal dementia (FTD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Alzheimer's disease (AD)) TAU is recognized as a significant pathogenic driver of the disease. In many secondary tauopathies, including Parkinson's disease (PD) and Huntington's disease (HD), TAU is suggested to contribute to the development of dementia, but in others (e.g. Niemann-Pick disease (NPC)) TAU may only be a bystander. The genetic and pathological mechanisms underlying TAU pathology are often not fully understood. In this review, the genetic predispositions and variants associated with both primary and secondary tauopathies are examined in detail, assessing evidence for the role of TAU in these conditions. We highlight less common genetic forms of tauopathies to increase awareness for these disorders and the involvement of TAU in their pathology. This approach not only contributes to a deeper understanding of these conditions but may also lay the groundwork for potential TAU-based therapeutic interventions for various tauopathies.

New knowledge on anti-IgLON5 disease

PURPOSE OF REVIEW

Anti-IgLON5 disease is characterized by a distinctive sleep disorder, associated with a heterogeneous spectrum of neurological symptoms. Initial autopsies showed a novel neuronal tauopathy predominantly located in the tegmentum of the brainstem. Recently, new diagnostic red flags, biomarkers predictors of response to immunotherapy, and novel insights into the autoimmune pathogenesis of the disease have been reported.

RECENT FINDINGS

Patients with diagnosis of neurodegenerative dementia, progressive supranuclear palsy (PSP) or with motor-neuron disease (MND)-like syndrome have been reported to have IgLON5 antibodies, which are the hallmark of anti-IgLON5 disease. Second, low levels of neurofilament light chain in serum and cerebrospinal fluid of patients at disease onset could be a predictor of immunotherapy response. Recent neuropathological studies indicate that the neuronal tau deposits occur late in the course of the disease. Moreover, IgLON5 antibodies induce cytoskeletal changes in cultured hippocampal neurons suggesting that the tauopathy could be secondary of the IgLON5 antibody effects.

SUMMARY

Anti-IgLON5 disease can mimic and should be considered in atypical presentations of MND, neurodegenerative dementia and PSP. Neurofilament light chain levels seem promising biomarker for disease prognosis. Finally, the neuropathological and in vitro experimental studies strengthen the autoimmune hypothesis of the disease.

Neuropathological assessment of the olfactory bulb and tract in individuals with COVID-19

The majority of patients with Parkinson disease (PD) experience a loss in their sense of smell and accumulate insoluble α-synuclein aggregates in their olfactory bulbs (OB). Subjects affected by a SARS-CoV-2-linked illness (COVID-19) also frequently experience hyposmia. We previously postulated that microglial activation as well as α-synuclein and tau misprocessing can occur during host responses following microbial encounters. Using semiquantitative measurements of immunohistochemical signals, we examined OB and olfactory tract specimens collected serially at autopsies between 2020 and 2023. Deceased subjects comprised 50 adults, which included COVID19 + patients (n = 22), individuals with Lewy body disease (e.g., PD; dementia with Lewy bodies (n = 6)), Alzheimer disease (AD; n = 3), and other neurodegenerative disorders (e.g., progressive supranuclear palsy (n = 2); multisystem atrophy (n = 1)). Further, we included neurologically healthy controls (n = 9), and added subjects with an inflammation-rich brain disorder as neurological controls (NCO; n = 7). When probing for microglial and histiocytic reactivity in the anterior olfactory nuclei (AON) by anti-CD68 immunostaining, scores were consistently elevated in NCO and AD cases. In contrast, microglial signals on average were not significantly altered in COVID19 + patients relative to healthy controls, although anti-CD68 reactivity in their OB and tracts declined with progression in age. Mild-to-moderate increases in phospho-α-synuclein and phospho-tau signals were detected in the AON of tauopathy- and synucleinopathy-afflicted brains, respectively, consistent with mixed pathology, as described by others. Lastly, when both sides were available for comparison in our case series, we saw no asymmetry in the degree of pathology of the left versus right OB and tracts. We concluded from our autopsy series that after a fatal course of COVID-19, microscopic changes in the rostral, intracranial portion of the olfactory circuitry -when present- reflected neurodegenerative processes seen elsewhere in the brain. In general, microglial reactivity correlated best with the degree of Alzheimer's-linked tauopathy and declined with progression of age in COVID19 + patients.

Clinical analysis and literature review of two paediatric cases of anti-IgLON5 antibody-related encephalitis

Introduction

Anti-IgLON5 antibody-related encephalitis is a rare autoimmune disorder of the central nervous system, predominantly occurring in middle-aged elderly individuals, with paediatric cases being exceptionally rare. This study aims to enhance the understanding of paediatric anti-IgLON5 antibody-related encephalitis by summarising its clinical and therapeutic characteristics.

Method

A retrospective analysis was conducted on two paediatric patients diagnosed with anti-IgLON5 antibody-related encephalitis at Hunan Children's Hospital from August 2022 to November 2023. This involved reviewing their medical records and follow-up data, in addition to a literature review.

Results

The study involved two patients, one male and one female, aged between 2.5 and 9.6 years, both presenting with an acute/subacute course of illness. Clinically, both exhibited movement disorders (including dystonia, involuntary movements, and ataxia), cognitive impairments, sleep disturbances, and psychiatric symptoms. Patient 1 experienced epileptic seizures, while Patient 2 exhibited brainstem symptoms and abnormal eye movements. Neither patient showed autonomic dysfunction. Patient 1 had normal cerebrospinal fluid (CSF) and Brain MRI findings, whereas Patient 2 showed moderate leukocytosis and mild protein elevation in the CSF, and Brain MRI revealed symmetrical lesions in the basal ganglia and cerebellum. Oligoclonal bands in the CSF were positive in both cases. Both patients tested negative for HLA-DQB*05:01 and HLA-DRB*10:01. They received both first-line and second-line immunotherapies, with Patient 2 showing a poor response to treatment.

Discussion

Paediatric cases of anti-IgLON5 antibody-related encephalitis similarly present sleep disturbances as a core symptom, alongside various forms of movement disorders. Immunotherapy is partially effective. Compared to adult patients, these paediatric cases tend to exhibit more pronounced psychiatric symptoms, a more rapid onset, and more evident inflammatory changes in the CSF. The condition appears to have a limited association with HLA-DQB*05:01 and HLA-DRB*10:01 polymorphisms.

Phenotypic Insights Into Anti-IgLON5 Disease in IgLON5-Deficient Mice

BACKGROUND AND OBJECTIVES

Anti-IgLON5 disease is an autoimmune neurodegenerative disorder characterized by various phenotypes, notably sleep and movement disorders and tau pathology. Although the disease is known to be associated with the neuronal cell adhesion protein IgLON5, the physiologic function of IgLON5 remains elusive. There are conflicting views on whether autoantibodies cause loss of function, activation of IgLON5, or inflammation-associated neuronal damage, ultimately leading to the disease. We generated IgLON5 knockout (-/-) mice to investigate the functions of IgLON5 and elucidate the pathomechanism of anti-IgLON5 disease.

METHODS

IgLON5 knockout (-/-) mice underwent behavioral tests investigating motor function, psychiatric function (notably anxiety and depression), social and exploratory behaviors, spatial learning and memory, and sensory perception. Histologic analysis was conducted to investigate tau aggregation in mice with tauopathy.

RESULTS

IgLON5-/- mice had poorer performance in the wire hang and rotarod tests (which are tests for motor function) than wild-type mice. Moreover, IgLON5-/- mice exhibited decreased anxiety-like behavior and/or hyperactivity in behavior tests, including light/dark transition test and open field test. IgLON5-/- mice also exhibited poorer remote memory in the contextual fear conditioning test. However, neither sleeping disabilities assessed by EEG nor tau aggregation was detected in the knockout mice.

DISCUSSION

These results suggest that IgLON5 is associated with activity, anxiety, motor ability, and contextual fear memory. Comparing the various phenotypes of anti-IgLON5 disease, anti-IgLON5 disease might partially be associated with loss of function of IgLON5; however, other phenotypes, such as sleep disorders and tau aggregation, can be caused by gain of function of IgLON5 and/or neuronal damage due to inflammation. Further studies are needed to elucidate the role of IgLON5 in the pathogenesis of anti-IgLON5 diseases.

Digestive and Absorptive Properties of the Antarctic Krill Tripeptide Phe-Pro-Phe (FPF) and Its Auxiliary Memory-Enhancing Effect

Aging and stress have contributed to the development of memory disorders. Phe-Pro-Phe (FPF) was identified with high stability by mass spectrometry from simulated gastrointestinal digestion and everted gut sac products of the Antarctic krill peptide Ser-Ser-Asp-Ala-Phe-Phe-Pro-Phe-Arg (SSDAFFPFR) which was found to have a positive impact on memory enhancement. This study investigated the digestive stability, absorption, and memory-enhancing effects of FPF using nuclear magnetic resonance spectroscopy, simulated gastrointestinal digestion, in vivo fluorescence distribution analysis, mouse behavioral experiments, acetylcholine function, Nissl staining, immunofluorescence, and immunohistochemistry. FPF crossed the blood-brain barrier into the brain after digestion, significantly reduced shock time, working memory errors, and reference memory errors, and increased the recognition index. Additionally, FPF elevated ACh content; Nissl body counts; and CREB, SYN, and PSD-95 expression levels, while reducing AChE activity (P < 0.05). This implies that FPF prevents scopolamine-induced memory impairment and provides a basis for future research on memory disorders.

Pathophysiological Effects of Autoantibodies in Autoimmune Encephalitides

The heterogeneity of autoantibody targets in autoimmune encephalitides presents a challenge for understanding cellular and humoral pathophysiology, and the development of new treatment strategies. Thus, current treatment aims at autoantibody removal and immunosuppression, and is primarily based on data generated from other autoimmune neurological diseases and expert consensus. There are many subtypes of autoimmune encephalitides, which now entails both diseases with autoantibodies targeting extracellular antigens and classical paraneoplastic syndromes with autoantibodies targeting intracellular antigens. Here, we review the current knowledge of molecular and cellular effects of autoantibodies associated with autoimmune encephalitis, and evaluate the evidence behind the proposed pathophysiological mechanisms of autoantibodies in autoimmune encephalitis.

Purified Serum IgG from a Patient with Anti-IgLON5 Antibody Cause Long-Term Movement Disorders with Impaired Dopaminergic Pathways in Mice

Background: Anti-IgLON5 disease is a rare autoimmune disease of the central nervous system. It typically manifests as a chronic condition, characterized by cognitive impairments, movement disorders, and sleep disorders. The mechanisms underlying movement disorders in this disease remain poorly understood due to a lack of research. Furthermore, this disease exhibits both neuroimmune and neurodegenerative characteristics. The objective of this study is to explore the underlying mechanisms of movement disorders caused by anti-IgLON5 antibodies for the first time. Methods: Antibodies were purified from the serum of a confirmed patient of anti-IgLON5 disease. The passive transfer animal models were employed, where antibodies were continuously injected into the substantia nigra pars compacta (SNc) of the mouse midbrain using stereotactic injection to explore the mechanism of movement disorder. The effects of anti-IgLON5 antibodies on dopaminergic neurons in the SNc and neurodegeneration were examined through immunohistochemistry. Changes in neurotransmitter levels in the basal ganglia were assessed using high-performance liquid chromatography. Additionally, RNA-seq was employed to identify the differentially expressed genes associated with the short-term and long-term effects of anti-IgLON5 antibody on the SNc. Results: Mice injected with anti-IgLON5 antibodies in the SNc exhibited persistent movement impairments for up to 3 months. One week after antibody injection, the number of TH neurons significantly decreased compared to the control group, accompanied by reduced projection fibers in the basal ganglia and decreased dopamine levels. After 3 months of antibody injection, an increase in phosphorylated Tau was observed in the SNc of the midbrain. Additionally, long-term sustained activation of microglia was detected in the SNc. The differentially expressed genes of long-term effects of IgLON5 antibodies were different from their short-term effects on the SNc. Conclusion: Purified serum IgG from a patient with anti-IgLON5 antibodies can cause long-term movement disorder in mice. The movement disorders appear to be linked to the impaired dopaminergic pathway, and the increased p-Tau showed neurodegenerative changes induced by the anti-IgLON5 antibody.

Case report: Anti-IgLON5 disease and anti-LGI1 encephalitis following COVID-19

Anti-IgLON family member 5 (IgLON5) disease is a rare autoimmune encephalitis, characterized by sleep problems, cognitive decline, gait abnormalities, and bulbar dysfunction. Anti-leucine-rich glioma-inactivated 1 (LGI1) autoimmune encephalitis is characterized by cognitive dysfunction, mental disorders, faciobrachial dystonic seizures (FBDS), and hyponatremia. Various studies report that coronavirus disease 2019 (COVID-19) have an effect on the nervous system and induce a wide range of neurological symptoms. Autoimmune encephalitis is one of the neurological complications in severe acute respiratory syndrome coronavirus 2 infection. Until now, autoimmune encephalitis with both anti-IgLON5 and anti-LGI1 receptor antibodies following COVID-19 is rarely reported. The case report described a 40-year-old man who presented with sleep behavior disorder, daytime sleepiness, paramnesia, cognitive decline, FBDS, and anxiety following COVID-19. Anti-IgLON5 and anti-LGI1 receptor antibodies were positive in serum, and anti-LGI1 receptor antibodies were positive in cerebrospinal fluid. The patient presented with typical symptoms of anti-IgLON5 disease such as sleep behavior disorder, obstructive sleep apnea, and daytime sleepiness. Moreover, he presented with FBDS, which is common in anti-LGI1 encephalitis. Therefore, the patient was diagnosed with anti-IgLON5 disease and anti-LGI1 autoimmune encephalitis. The patient turned better after high-dose steroid and mycophenolate mofetil therapy. The case serves to increase the awareness of rare autoimmune encephalitis after COVID-19.

The anterior cingulate cortex contributes to the analgesic rather than the anxiolytic effects of duloxetine in chronic pain-induced anxiety

Mood disorders, such as anxiety and depression, are commonly found in people suffering from chronic pain. Serotonin-norepinephrine reuptake inhibitors (SNRIs) are potential in alleviating chronic pain and are the first-line option for anxiety disorder. The anterior cingulate cortex (ACC) plays a vital role in chronic pain-induced anxiety, but its role in the therapeutic effects of SNRIs remains largely unclear. We used complete Freund's adjuvant (CFA) in this current study to induce chronic inflammatory pain. Von Frey test was used to measure the mechanical withdrawal threshold. The elevated plus maze test (EPM) and the novelty-suppressed feeding test (NSF) were used to measure anxiety-like behaviors. Twenty-one days after the modeling, anxiety-like behaviors were successfully induced in CFA mice, and a 3-day intraperitoneal injection of duloxetine attenuated such behaviors. While, mechanical hyperalgesia was also improved. Then, we locally infused duloxetine in ACC for 3 days only to find out its analgesic effect in CFA mice. Furthermore, we used fiber photometry to discover decreased glutamatergic excitability and enhanced serotonin concentration in ACC after intraperitoneal injection of duloxetine. Overall, this study proposed a potential mechanism for the analgesic effect of duloxetine and shed light on further studies on the mechanism of its anxiolytic effect in chronic pain-induced anxiety.

Anti-IgLON5 disease: a novel topic beyond neuroimmunology

Anti-IgLON5 disease is a recently defined autoimmune disorder of the nervous system associated with autoantibodies against IgLON5. Given its broad clinical spectrum and extremely complex pathogenesis, as well as difficulties in its early diagnosis and treatment, anti-IgLON5 disease has become the subject of considerable research attention in the field of neuroimmunology. Anti-IgLON5 disease has characteristics of both autoimmunity and neurodegeneration due to the unique activity of the anti-IgLON5 antibody. Neuropathologic examination revealed the presence of a tauopathy preferentially affecting the hypothalamus and brainstem tegmentum, potentially broadening our understanding of tauopathies. In contrast to that seen with other autoimmune encephalitis-related antibodies, basic studies have demonstrated that IgLON5 antibody-induced neuronal damage and degeneration are irreversible, indicative of a potential link between autoimmunity and neurodegeneration in anti-IgLON5 disease. Herein, we comprehensively review and discuss basic and clinical studies relating to anti-IgLON5 disease to better understand this complicated disorder.